1. Field of the Invention
The present invention concerns the digital processing of images, for the obtaining of special geometrical effects. The processing is done using an original image formed by digital signals, representing points distributed along the intersections of a rectangular grid formed by rows and columns. The processing consists in modifying the geometry of the original image to obtain, for example, the appearance of a projection of the image on a curved surface, or a reduction, namely a simple change in scale of the image.
In a known method to achieve these special effects, the conversion grid, the rows and columns of which can be curved, is determined, this grid is superimposed on the original rectangular grid, and their points of intersection are determined by interpolation, using the neighboring points on the original grid; the points of the conversion grid are then carried over to a rectangular grid, having the same number of rows and columns and the special effect is obtained. To correct aliasing phenomena, namely the conversion of high frequencies of the spectrum into low frequencies, a known method lies in determining each point of intersection by bringing into play two factors, called local compression factors, equal to the respective norms of the vectors respectively tangential to the row and column at the point considered.
It must be noted that the smaller the final image as compared with the original image, the greater is the number of neighboring points that are to be taken into account in determining, by interpolation, the points of the new grid in order to minimize the aliasing phenomena. Thus, when the final image is smaller than the original image, the prior art devices require long computation periods to prevent the aliasing phenomenon.